text:
Why do composite materials not decrease the OEW
Boeing states in an article, that the 787 Dreamliner saves around 20% of weight in a comparison to an aluminium based aircraft. Boeing 787 Dreamliner
When comparing the 787 Dreamliner to other "recently" introduced aircraft such as the A330-900 it does not seem that any weight has been saved. The A330-900 is an aluminium aircraft built on the A330-300.

Aircraft
OEW
MTOW
Passenger Exit Limit
OEW/MTOW
OEW/Exit Limit

B787-10
135t
254t
440
0.531
0.3068

A330-900
135t
251t
460
0.537
0.2935

When comparing the OEW to the MTOW it seems, that both values are pretty similar. Given the same OEW, the A330-900 can carry 20 passengers more.
What could be the reason, why the A330-900 has a same structural efficiency as the 787 Dreamliner, despite not using any composites? Doesn't the 787 Dreamliner should have decreased the OEW by 20% hence having a much better ratio of OEW/Exit Limit?

Very much related: What is the average weight difference between composite and metal airliners?. It probably answers your question.

If anybody wants to flag or vote to close as a duplicate, that would be good. I won't because that would mod-hammer it.

Does this answer your question? What is the average weight difference between composite and metal airliners?

This does not quite answer my questions, as it indicates, that weight savings from composites were used to build larger wings. Comparing the B787 and the A330-900 this is actually the opposite. The A330-900 has a 4m larger wingspan, using aluminium and still having a better structural efficiency.

Boeing's relationship with truth isn't not akin to an open marriage.

Does this answer your question? Why has structural efficiency (OEW / MTOW) not improved despite increasing use of carbon fiber?

Composite fuselage weight savings aren't as good as a straight beam-to-beam comparison indicates. There are design constraints other than structural load, which limit what can be achieved.
Meanwhile, Airbus wasn't just doing nothing, small improvements do get implemented even on existing models of aircraft.
The 787-10 is a slightly larger aircraft than the A330-900. It's got fewer exits, but through an extra 9" in width and a longer cabin, packs more seats in practice.
It's hard to compare apples to apples, because airlines don't operate two competing types at once. That said, the 787-10 tends to pack up to 310-344 seats in long haul, while the A330-900 typically does 290-350 (various airlines). The 787-10 also carries up to 40 LD3 containers vs 33 on the A330-900.
So the few percent of improvement from composites is used to get space for more cargo and pax, at the expense of somewhat less range compared to the A330.

Another factor is that the knowledge of the theoretical structural savings tends to cause weight creep on other areas, as an almost subconscious relaxing of weight control. Sort of a moral hazard effect combined with "death by a thousand cuts" as tiny weight increments in a thousand different places add up.


text:
What type of engine sound suppressors are used in general aviation?
Today we were discussing about noise pollution and why aircraft engines considered one of the biggest contributors to producing high intensity sound and I thought are there any ways and/or methods that manufacturers use on engines to control or lower the sound their engines are producing?

One place to look for info on engine noise reduction is right here on Aviation!

Piston engines as used in general aviation typically have a muffler (just like cars use) made of welded tubing. But you can't put mufflers on gas turbines because unlike a piston engine, a turbine does not create impulse noise which a muffler can filter out. Instead it creates an intense roaring noise caused by air being turbulently pumped through the engine at very high speed and then shot out the tailpipe with tremendous force.
Also note that a gas turbine engine is stuffed full of axial fan discs with many, many blades on each and when they are set to spinning at tens of thousands of RPM, they generate a siren-like scream that is impractical to suppress in an aircraft.
Note that the propulsive effect of a turbine engine is entirely based on its ability to accelerate extremely large mass flows of air mixed with hot combustion products and blow them out the exhaust. In this case, more thrust means more noise unless you can cleverly surround the hot core exhaust with a blanket of slower-moving bypass air from the fan disc.

Thank you for the response. If I may, you would know any thing about a component called the unit mixer? Typically, it is used on a gas turbine engine especially at the exhaust nozzle. I wasn't able to hear it properly when this other guy was mentioning it to a fellow student.

No, I don't, sorry. -NN

You describe turbojet engines here but those are obsolete.  Modern high-bypass turbofans develop almost all their thrust from the fan, not the turbine - they don't even bother putting a thrust reverser on the turbine part of thrust.   So there would be no particular loss of thrust from routing turbine output through muffling devices.

Almost all thrust? not 40-60%? and what muffler technology would you use to filter out 15-20 kilohertz blade noise? as it is, blanketing the core exhaust with the bypass works remarkably well.

From my understanding the turbo-fan or high-bypass engines are used in general aviation while a pure get is used in the military( example the F-35). In regards to the turbo-fan I have noticed that PW has developed an engine called the gear turbo-fan engine to mitigate sound pollution. I saw a picture of these mixer unit on the exit nozzle of a jet engine and I was amazed. You might be right this technology may have become obsolete. I understand that the fan is driven by the LP compressor which is connected to the LP turbine and the bypass air contributes about 80% to the thrust.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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Will a FAA 3rd class medical ever be issued to an airman on stimulants?
While the FAA's Do Not Issue list includes all controlled substances (including Stimulants), I am wondering if there are ever exceptions made.
I would like to fly but I am currently on Concerta (methylphenidate), which would certainly disqualify me without any special issuances or approvals made. Is it possible to be approved for a class 3 (or higher) medical, while on stimulant medications?

Will it ever? We can't know that.

No, these medications are disqualifying. The FAA requires that you stop them for 90 days before applying.
Assuming they were prescribed for an ADHD diagnosis, you will need to take a drug test to confirm that you are not medicated and then pass an intensive cognitive test. It's not a quick, easy, or cheap process, unfortunately.
If you were never diagnosed with ADHD, or were prescribed them for some other reason, you still will need to stop taking them. But not having an ADHD diagnosis on your record may make the process much easier. Carefully read exactly what you are required to disclose.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What are the flight formulas for aircraft
I am making a 2D flight "simulator" (trying to be as realistic as you can for 2d) and I need flight formulas for lift, speed and pitch. I have tried a few sites but I do not understand what they are saying. Can someone please tell me the answer (if this is needed i am using the Boeing 787-8 dreamline). Sorry if this post is not formatted/written properly this is my first time using this site.

What you're looking for is termed "flight dynamics" and takes up an entire book, like this one for example. Out of curiosity, how are you going to solve the equations of motion?

Is there a simple formula?

Can you explain what a 2D flight simulator consists of?  (i.e. which two dimensions?)   I'm picturing an overhead view, Instrument/Navigation trainer with a moving map type display.  If this is the case you should be able to greatly simplify the underlying code vs 3D.

Oh sorry i mean like side view not overhead.

Your '2D' problem (altitude, downrange) is really a 3DOF problem, (adding a rotation -- pitch angle).
Normal 3D flight is a 6DOF problem -- 3 positions, 3 angles.
You will need to write the equations for the accelerations of each DOF and then integrate them twice to the velocities and positions.  This is a coupled 2nd order ordinary differential equation.  You will start at an initial position and time-step your way through the solution.
You need control variables (pilot inputs).  These will normally be throttle and elevator angle.  If the aircraft is a glider, then you only have elevator angle.
A big decision is whether you want to represent the aircraft as a point mass, or as a distributed mass.  The point-mass assumption will set the pitching moment of inertia to zero.  This will make the angular equation of motion respond instantly.  You can then solve the pitch equilibrium equation as a statics equation (trim) or you can ignore it and treat either angle of attack or CL as your pitch input directly.
I would recommend you start with an even simpler problem.  A point-mass with no thrust and no drag (and no lift).  I.e. just mass.  This is the cannonball problem.  You can derive the equations of motion the same way and then implement an ODE solver to plot out the path of the cannonball given an initial Vx,Vy.
Once you have the cannonball working, I would add drag and then thrust, then lift.  When you add thrust, you can add a throttle control.  When you add lift, you will need to add the pitch control of some sort.
I would start with point-mass equations (ignore the pitch inertia) and only add it if when you get finished with everything else, you don't feel it is realistic enough.


text:
Alternatives to gabapentin acceptable to the FAA
I have neuropathy. Blood sugar under control, backbone seems to be okay—my doctors aren’t sure why I have it, but it hurts A LOT, especially at night. It is frequently severe. Am on gabapentin, FAA does not allow it.
Does anyone else with similar condition know of an acceptable alternative—even surgical—to gabapentin?

The guide for AMEs on neuropathy can be found here. It indicates that the AME can issue if they determine the condition is under control, your medications are acceptable, and that you have no symptoms that would interfere with flight duties.
The FAA does not publish a list of acceptable medications. You could consult with an AME and ask them what medications they would find acceptable. Note: you want a consultation. Don't fill out the medical form on MedXPress until you have discussed it with an AME beforehand! Once the actual medical process is started, the AME has to finish the process within two weeks. You will want all your ducks in a row before you start the process.

I don't think there are any medications you can take that are acceptable to the FAA.
I did a search on the FAA medication database.  The screenshot of the results are below.

What does "NOT ALLOWED. Requires 36 hr wait after last dose" even mean? Is it allowed as long as you wait 36 hours before flying?

Also worth noting that this list is unofficial. The FAA doesn't publish a list of acceptable medications.

As Chris says, putting that link up as "FAA medication database" is misleading to say the least

I'm assuming you have diabetes as you mentioned blood sugar being under control. In this case the diabetes itself is most likely the root cause for the neuropathy. Diabetic neuropathy cannot be reversed, it can only be treated with medication.
As you may well be aware, the possible reasons for neuropathy may be nerve pinch and other physical irritation to nerves, or a persistent inflammation. Also, in some cases people respond to certain diets very strongly, causing neuropathy or neuropathy like symptoms.
If doctors have already investigated all of these lines I'm sorry to say I have very little to offer. I've had neuropathy which was caused by my body having difficulties adapting to new mechanics, so to say, after a corrective surgery. Weight loss, exercise and physiotherapy did the trick for me.
Hoping for the best for you

@TYork I also endorse what said by Jpe61: weight loss (included a drastic reduction of carbohydrates and sugars replaced with seasonal fruits and vegetables), exercise and physiotherapy. All the best


text:
What does 3 blinking red lights on a small aircraft indicate?
I live by Lackland Air Force Base in San Antonio, Tx and have lived here 18 years and was also Airborne in the Ay and I just saw a small private aircraft fly above me with 3 flashing lights with 2 on each wingtip and one underneath the aircraft.  Before the aircraft flew over my house a helicopter fly over as well with 2 flashing white lights underneath it. What do these flashing red lights mean since I’ve never seen those lights before.  Normally they have their anti-collision lights on but not this time. Air traffic above my house seems out of the ordinary lately

I understand the colors on each wingtip for the purpose of direction of flight and anti-collision being red, green and white on aircraft but my question is why all red this time? You wouldn’t be able to distinguish if the aircraft is going left to right or right to left or coming straight forward. The helicopter had 2 white strobe lights instead of the green and red. I’m no aviation expert but I have been around this airspace for almost 20 years. I really didn’t understand if I had a reference statement. Thank you

The time of night was around 9:30 pm. I’m sorry if I confused anyone but I meant to say there was a red light on each wingtip and one one the underside of the plane so that totals 3 red lights with no additional lights. They were not the usual anti collision lights of red and green.  On the helicopter, there was 2 blinking WHITE lights underneath the helicopter.  Hopefully this clears up my question.  If images were allowed to be uploaded on this site I could have done so to clear any confusion

The blinking wingtip and fuselage strobe lights serve as anti-collision warnings, so pilots can more easily spot other planes in their vicinity, even in full daylight.
All small helicopters have two red lights on them so you can tell it's a helicopter from the ground at night.


text:
Adding fixed wing CFII SEL to helicopter CFII rating
I am a CFII, and have just been asked to assist a Helicopter pilot who has FIH (CFFII-Helicopter) and a Commercial fixed wing (SEL) certificates to obtain a CFII SEL rating.
My research to date indicates to me that he needs the following:

Spin training

Sufficient instruction to be able to teach required skills from the right seat of a single-engine airplane

Knowledge test-- AIF (short instrument add-on for FIH to CFII)

Does he also need to take the AFA knowledge test (add-on test for FRH to CFI)?

So, basically, he just wants to add on an airplane single engine and instrument airplane ratings onto his flight instructor certificate? Correct?


text:
How does air passing through the secondary heat exchanger in an aircraft's AC pack get cooled if ram air is not available/insufficient?
For example, if the aircraft is on ground and bleed air for the AC pack is provided by the APU.
Most explanations I've seen say that the air passing through the secondary heat exchanger in between the ACM's compressor and turbine stages is cooled by ram air, so I'm not sure how it would work in the scenario I mentioned above.

Does this answer your question? What's inside the packs? (Airbus a320) From the accepted answer: "The impeller fan creates cooling air flow when airspeed is low."

Thank you, it does!


text:
Confusing Missed Approach Instructions
The Missed approach Instructions for the VOR approach at Prescott AZ  (KPRC) KPRC VOR Rwy 12 reads as follows:

MISSED APPROACH: Climbing left turn to 9000 via DRK VORTAC R-305 then
right turn direct DRK VORTAC and hold, continue climb-in-hold to 9000.

If performing this MA, when you get to DRK VORTAC, and you are still not yet at 9000 MSL, One of three things must be true.

You are to continue outbound on the DRK 305 radial until you reach 9000, then turn right direct to DRK VORTAC and enter the holding pattern.
When reaching Drake, enter the holding pattern and continue climbing in the holding pattern to 9000 MSL.
This Missed Approach procedure, as written, is inconsistent and is erroneous. You cannot both execute a climbing left turn to 9000 via the drake 305 radial, and Climb in the holding pattern to 9000.

... or does the phrase "Climbing left turn to 9000... " at the beginning of the Missed Approach instructions simply mean that the final altitude of your climb will be 9000, not that you have to get there before the next step which reads "... then right turn direct DRK VORTAC ... "
This post from askacfi.com is interesting ...
More info (FWIW):   Submitted
Since posting this, I have contacted a FSDO, and was told that they cannot offer definitive official help on this, but that I should contact the controlling ATC agency, which in this case is Phoenix TRACON. They did agree that the MAP instructions, as written, are confusing and probably should be clarified.
So I contacted and spoke with a controller at PHX TRACON. He also agreed that the procedure is confusing, but told me what he expected a pilot to do when executing this procedure "as published". He said he expects pilots to turn left and roll out on heading DIRECT to the DRK VORTAC, (NOT to intercept the 125 radial inbound to DRK), then, at station passage, turn right to track outbound on the DRK 305 radial, climbing until they reach 9000 MSL. Then, when reaching 9000 MSL, but not before, turn right direct back to the VORTAC, and enter the holding pattern. When I pointed out the inconsistency of the last six words, "continue climb-in-hold to 9000", he said I should just ignore that.  When I asked him if he could tell me who is responsible for these procedures, (who I might call to pursue this further), he did not know, and could not tell me who to contact next.
Also worth reviewing is this FAA Doc, which outlines the latest info on this procedure, including history of the changes to it.
Final Entry... I submitted an aeronautical inquiry about this procedure, and was pleasantly surprised to get a response almost immediately. As you can read below, their intent is almost identical to my option 1, except that they want you to roll out of the first turn Direct to DRK VORTAC.
Their corrected text will, (once flight test has been accomplished and approved) read as follows:
climbing left turn to 9000 direct DRK VORTAC and DRK R-305, then right turn direct DRK VORTAC and hold
Here is their complete response:

Charles Bretana,
Your Aeronautical Inquiry has been closed.  If you have followup
questions or comments, please use the online inquiry form instead of
sending via email.
======================================================== FAA Response:
We agree that, as written, the missed approach instructions are
confusing.  We also agree that the climb on R-305 then reverse course
and continue climb in hold are not compatible. Here's our 1996, Amdt 2
instructions: CLIMBING LEFT TURN TO 9000 DIRECT DRK VORTAC, CONTINUE
CLIMB TO 9000 NW BOUND VIA DRK R-305, THEN TURN RIGHT DIRECT DRK
VORTAC AND HOLD.  The Flight Inspection Pilot who flew the amendment,
had us change the instructions to what was charted prior to Amdt 3.
What was observed at the time was that the missed approach required
two turns, which our criteria does not permit without a waiver. He
also felt that the turn direct the facility may be too great a turn
(not standard) for some aircraft. Our amendment 3 added continue
climb-in-hold without an explanation. We will request our Development
Team to change the instructions to: climbing left turn to 9000 direct
DRK VORTAC and DRK R-305, then right turn direct DRK VORTAC and hold.
So, you would climb direct DRK, intercept R-305, at 9000 reverse
course, proceed direct DRK and hold.  We consider that a continuance
rather than two turns, but the change will require another Flight
Inspection of the procedure.  Hope this is acceptable, and thank you
for bringing this to our attention.

Thanks to all for your comments and input!

The addition of the "continue climb-in-hold" is   a very recent change; as of the last cycle the MA read MISSED APPROACH: Climbing left turn to 9000 via DRK VORTAC and DRK R-305, then turn right direct DRK VORTAC and hold. The controller you talked to may not have fully understood this, nor the reason for it, but it appears the TERPS guys very much want you to turn back to DRK before reaching 9000.

Anyway, to answer your last question: Mr. John Bordy (FAA/AJV-A422) is the individual responsible for this most recent change, as indicated on the previously linked Form 8260-3.

I noticed that, and I have submitted what us called an  "Aeronautical Inquiry" on this which I believe will be routed to Mr Bordy...

As noted in my most recent edit to the question, I have a response from FAA, who agreed with our joint comments, and are in the process of amending the text of the MAP procedures to make it (almost) compatible with option 1.

Interesting comment in FAAs response, (that I have just learned for the first time), is that ".... that the missed approach required two turns, which our criteria does not permit without a waiver."

I think they still got it wrong...

@Jpe61,  I tend to agree with you, but I do not as yet know (and they did not explain), why they do not want the aircraft entering holding at less than 9000 MSL. I would like to know the answer to that question before I concurred with you 100%. A friend suggested that climbing to a target altitude in a holding pattern is significantly busier (more difficult), and might be a task they simply attempt to avoid. Not sure I buy that, but ....

Now that I think of it, climbing in holding is very, very counterintuitive... The principle in holding patterns is to fill from the top.

@Jpe61, Ahhhh, yes, that's right, good point. And on this specific approach that holding pattern is indeed the same hold used by incoming (newly arriving), aircraft using the DRK IAF & Procedure Turn option.

My two cents say that your option 2 is the correct execution.

Climbing left turn to 9000 via DRK VORTAC R-305 then right turn direct DRK VORTAC and hold, continue climb-in-hold to 9000

emphasis mine
The to 9000 is presented right at the beginning just for good measure, so that the pilot knows what to "aim" for. Procedure is then flown as described: intercept R-305, turn right towards DRK, and enter the holding pattern (parallel entry). If 9000ft is not reached by then, continue climb in pattern.
To further clarify the instructions, I've emphasized part of the text in the quote above: "Climbing ... to 9000 via DRK...". Note that if the missed approach was to be executed such that 9000ft shall be reached before DRK, the wording would be different. The "via" would be omitted, and as DJClayworth pointed out, there would be no need to reference the altitude later on in the description of the procedure. Also the beginnig should be something along the lines "Circle left climbing to 9000 , then intercept DRK VORTAC R-305...". Note that I'm not aware of such procedures existing  but my knowledge of airports around the world is very, very limited.
As you can see in the bottom part of te plate, the missed approach is also described there with three boxes:
1st one sets target altitude and left turn towards R-305 DRK
2nd one commands right turn
3rd one sets DRK as target
9000ft might seem like a lot, but since field elevation is 5045ft, it's really not that much.
I agree that this procedure is not describe in the clearest possible manner, allthough it might be the most concise way to do it.

I think you're correct, but I think this way of phrasing MA instructions is confusing and misleading ...

Oops, of course @757toga

Just adding that after some significant research and phone calls, I am now no longer certain what the correct interpretation should be.

@CharlesBretana I've edited my answer with further points supporting my view on this matter.

Yes, I agree. The point that some are using to argue otherwise is that the 305 Radial only exists NW of the VORTAC. In their view, to the SE, its the 125 radial. In my view the flight path that results from that interpretation,  (overflying DRK twice), simply does not make sense.

I somewhat agree @757toga so I also slighthly disagree with my own answer, but that's how I mostly see this matter. The wording is not all that unclear ro me, but could be better. Apropos, I checked some European approach plates, they all seemed to omit the "and hold" part from the description, even though the MA clearly had one as the endpoint.

I have received a response to my FAA Aeronautical Inquiry about this. See added text in my question above... But bottom line is they are changing the text to read  [climbing left turn to 9000 direct DRK VORTAC and DRK R-305, then right turn direct DRK VORTAC and hold], (without the continue climb-in-hold to 9000), so my option 1 (contrary to what I initially thought), most closely matches what their intent is.

Option 2 is clearly correct.
This is because it says "Continue climb-in-hold to 9000". If you were expected to be at 9000 before entering the hold at DRK there would be no need to add that statement.


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Mode-C & ADS-B Out rings around class Bravo airspace
My ADS-B Out unit (and EchoUAT), is broken, (wifi out) and I need to send it back to be repaired/replaced. While it is out, I will only have a legacy Transponder with Mode-C.
Can I fly over (above) class bravo airspace?
What about under the depicted airspace shelves, inside the Class Bravo 30nm ring but below the minimum altitude depicted on the chart for the shelf I am under?
The documentation I have found on the internet does not make it clear that I need both systems (Mode-C and ADS-B, or just one or the other to be compliant inside the Mode-C Veil.
If I need both, then I need to avoid the 30 nm ring completely, at any altitude. and while the box is being repaired I cannot fly to or from any airports under the Bravo airspace shelves.

14 CFR 91.225(d):

After January 1, 2020, except as prohibited in paragraph (i)(2) of this section or unless otherwise authorized by ATC, no person may operate an aircraft in the following airspace unless the aircraft has equipment installed that meets the requirements in paragraph (b) of this section:

(2) Except as provided for in paragraph (e) of this section, within 30 nautical miles of an airport listed in appendix D, section 1 to this part from the surface upward to 10,000 feet MSL;

Breaking that down: You may not operate within 30 miles of a "Mode C veil" airport unless you have a working ADS-B Out setup, or unless your aircraft was originally certificated without an engine-driven electrical setup (and has not subsequently been so certificated). The "upward to 10,000 feet" is not a loophole because once you reach that altitude paragraph (d)(4) takes effect.
But there are two general "outs" that you have available to you: the prohibitions in paragraph (i)(2) and an ATC authorization.
Paragraph (i) incorporates by reference the technical standards for ADS-B Out. I'm not sure how an operation could be prohibited by that paragraph... maybe they're saying that if your equipment is out-of-standards then you aren't allowed to use it? It isn't clear.
But the ATC authorization is easier to understand: ATC may approve a deviation from any of the ADS-B Out requirements. The FAA has set up the ADS-B Deviation Authorization Preflight Tool (ADAPT) "to manage these authorization requests." In order to be considered for a deviation, you need to have a working Mode C transponder and you need to submit the request between 24 hours and 1 hour before your flight.
The ADAPT page does have a note saying that the FAA will not issue in-flight deviations nor will ATC facilities accept requests by telephone. If this is approved FAA policy then I have not been informed of it. FAA JO 7110.65 does include a paragraph instructing controllers how to handle requests to deviate from transponder/Mode C requirements between 10,000' and 18,000' (91.215), but I do not believe there is any guidance on dealing with ADS-B deviation requests (91.225) in either JO 7110.65 or JO 7210.3. So you could make your deviation request by telephone (make sure to call a recorded line!) or over the frequency to a controller, and they may or may not approve it. Using the ADAPT would probably provide a better paper trail for everyone involved, however.

https://www.faa.gov/air_traffic/publications/atpubs/foa_html/chap5_section_4.html
Better to be safe than sorry. If you can't comply with the Mode C veil or aren't sure (because this is a crazy big risk if unsure), you can call the approach phone number an hour before departure and you can coordinate and get permission to do the flight. They may require things like, a flight plan, being in communication with, a certain route, etc.
I forgot to mention but I'm not exactly sure of the details of your legacy transponder but if it's a 4096 with mode C and it's established to the aircraft you may be good to go. *To enter the mode C veil you need mode c. To enter the bravo you need Mode C and ADSB out.
https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F/part-91/subpart-C/section-91.215


text:
How does turn radius change with velocity between Vm and Vs, as in a 180 degree turn in a canyon?
I just heard a briefing on mountain flying, and it was stated that the first thing, or the most important thing to do in the event you are unfortunate enough to find yourself in a blind canyon with no safe way out straight ahead, [ and you need to turn around and go back the way you got in, is to pull power.
To address comments made in some of the answers below, I am asking about a situation in a General Aviation (GA) aircraft, where you do NOT have sufficient airspeed/aircraft performance to "go over the top" i.e., perform an Immelmann/Cuban Eight or Hammerhead.
It was stated that this is the recommended approach because the turning radius of the aircraft is dependent on velocity and therefore that the slower you are going the tighter the turn, and the less distance it will take to turn around. (I suspect this might be based on a simplistic reading of the Turn Radius formula $R= V^2 / G$, without considering that below Maneuvering Speed ($V_M$), $G$ also varies with $V^2$).

NOTE. By G, I mean the aircraft G, or Load Factor (engineers use the letter N), which is equal, generally, to Lift divided by
aircraft Weight, and specifically, in this issue,  the radial G, or that
horizontal component of the total aircraft G which is turning the aircraft, and not just keeping it in the air.

I have always thought just the opposite, (about the power setting), for a variety of reasons, and decided to do the physics/aerodynamics analysis and see what that predicted.
Without going through the Math, what I ended up with was that the turn radius in a maximum performance turn, (AOA at $C_{Lmax}$), would be
$$R \cong \frac{V^2}{\sqrt{V^2-V_s^2}}$$
where:

$R$ .... Turn radius
$V$....  Aircraft true air speed
$V_S$... Stall speed (TAS)

This is based on the assumption that we are below maneuvering speed ($V_M$, or what we called $V_c$ (Corner Velocity) in the USAF), so we are limited by stall AOA and not placard G-Limits, and that we need to maintain level flight or at least a controllable rate of descent. i.e., we need to maintain a bank angle no greater than that which would generate a vertical component of Lift sufficient to keep the nose from dropping any further. Therefore, the slower and closer to stall we get, the less angle of bank we can hold (and the less of our lift is actually turning the aircraft). As I remember this from the Air Force, turn radius is constant below $V_M$, and therefore, the most critical aspect of this issue is preventing stall, loss of control, and spinning into the ground. plus, keeping the airspeed as high as possible (below $V_m$) allows us to use the highest possible bank angle, where we will get the greatest possible horizontal component of wing lift to turn us around.
When I graph the above equation, I get what I expected, that below $V_M$, the turn radius is more or less constant, except that the closer to stall you get, the bigger it gets (which makes sense, at stall, you cannot turn at all!)  For an aircraft with a stall speed of about 59K, The graph looked like the below:  Vertical is turn radius (ft) and horizontal is true airspeed):

The kink in the graph at about 112Kts is because I assumed a $3.8 G$ aircraft, so $V_M$ would be $V_s \sqrt{3.8}$ or about 112 Kts. After that we are limited by placard G, and not AOA, and Turn Radius is just $V^2/G$.
So, if you notice, contrary to my expectations, the turn radius is not roughly constant below $V_M$, or, considering bank angle, gradually increasing the entire way as you slow from $V_M$ to $V_S$. No, it gradually decreases at first, (although not by a lot), until reaching a minimum about 25 kts above $V_s$, and then does as I expected, it rises asymptotically to the Stall speed line.
So, my question is, why, from a physical perspective does the turn radius first decrease as we slow from $V_M$ to $V_S$? Is there a physical explanation for this phenomena?
This does not affect the overall conclusion, (it still absolutely makes sense to add power to minimize airspeed bleed off and stay as far from  the stall as possible, rather than intentionally reduce power, slow down and risk stalling), but if I am going to explain this, then I should be able to explain why the curve behaves as it does from a pilot perspective.

You derived a formula yourself, and now you're asking for a physical explanation of your formula? If I'm understanding that correctly, I don't see how anyone besides you could reasonably answer your question. (Especially since the answer turned out to be "the derivation, which I haven't shown to you, has a mistake in it".)

Read the answer I posted below. You are right, the equation I had derived in the initial post was flawed. When I realized that, initially, I deleted the question, and was politely informed that best practice here was to leave it as originally posted so that others might benefit from the original question and the explanation of why it was flawed. I agree.

Man, I would never ever ever pull power in such a situation. I would pull UP, while firewalling the throttle.  Yes, I want the airspeed to go down-- but I would not try to accomplish that by pulling aft on the throttle control!

Slowing down, if already below maneuvering speed, is the most dangerous, and the absolutely worst possible thing to do. The slower you are, the less energy you have to turn, the larger your tightest turn radius will be, and most critically, the  closer you are to stall, spin and death.

NOTE:. I have resolved my question. I had made a mistake in the algebraic derivation of the Formula I was using to graph this. I have included both the old incorrect formula and the new corrected one, and displayed on the graph the curves resulting from each one.
Without going through the Math, I noticed that my equation was wrong:
WRONG FORMULA:  $$R \cong \frac{V^2}{\sqrt{V^2-V_s^2}}$$
CORRECT FORMULA:  $$R \cong \frac{V^2 V_s^2}{g (\sqrt{V^4-V_s^4})}$$
where:

$R$ .... Turn radius
$V$....  Aircraft true air speed
$V_S$... Stall speed (TAS)
$g$ .... 32.2 $ft/sec^2$

Once I modified the formula properly, the curve indicates exactly what you would expect, so my question is now moot.
Derivation:
Staring with the standard Turn radius formula:
1.
$$R \cong \frac{V^2}{G_R}$$
where $G_R$ is the Turning Lift (the horizontal component of the Lift vector), divided by the weight, referred to as Radial G, the G that is actually turning the aircraft and not just holding it up in level flight.
Since the Aircraft Load Factor or total G ($G_T$), radial G ($G_R$) , and the 1 G (god’s G) that is holding the aircraft up in the air form a 90 degree right triangle depicted in the diagram above, they must conform to Pythagoras Theorem that says that in a 90 degree triangle the square of the hypotenuse  must be equal to the sum of the squares of the other two sides. so,
$G_R^2 + {1g}^2 = G_T^2$
$G_R = \sqrt{G_T^2 - {1g}^2}$
Back to turn radius formula, substituting expression for $G_R$,

$$R \cong \frac{V^2}{\sqrt{G_T^2 - {1g}^2}}$$

Now total G ($G_T$) - assuming we establish the maximum available G at whatever airspeed we are at, is simply the maximum available Lift ($L = C_{Lmax} ρ V^2 S$), divided by the aircraft weight
$G_T = g\frac{C_{Lmax} ρ V^2 S}{W} $
Substituting this into our equation gives us

$$R \cong \frac{V^2}{\sqrt{{g^2\frac{(C_{Lmax} ρ V^2 S)^2}{W^2}} -
{1g}^2}}$$

Now here's the trick, The maximum available lift at stall speed $V_S$  is, by definition, equal to the aircraft weight, so
$W = C_{Lmax} ρ V_S^2 S$
Substituting this, and simplifying,

$$R \cong \frac{V^2}{g\sqrt{{\frac{(C_{Lmax} ρ V^2 S)^2}{(C_{Lmax} ρ
V_S^2 S)^2}} - 1}}$$

and cancelling,

$$R \cong \frac{V^2}{g\sqrt{{(\frac{V^2}{V_S^2})^2} - 1}}$$

Simplifying,

$$R \cong \frac{V^2}{g\sqrt{{\frac{V^4}{V_S^4}} -
\frac{V_S^4}{V_S^4}}}$$

and finally,

$$R \cong \frac{V^2 V_s^2}{g \sqrt{V^4-V_s^4}}$$

What are the units of your two axes?

Horizontal is true air speed (velocity). Doesn't matter what units. could be knots or mph. What matters is that the asymptote is the stall speed, and the kink is the maneuver speed. The vertical scale measures turn radius in feet. In the formulae, velocities must be in feet/sec, and turn radius is in feet, as  otherwise the g constant would have to scaled to match.

The advice to pull power before turning is only good if you cannot climb. What I learned is to pull up and then do the turn at low speed, but a higher altitude. In normal canyons, this should also give you more maneuvering space.
When flying gliders in the Alps, you learn the "Bayernkurve" (Bavarian turn). This is almost a hammerhead turn, but not flown vertically like a real hammerhead, but rather with the steepest climb angle possible. Flying in a steep climb will lower the load factor and help to make the turn tighter.
Not only should speed be as low as the load factor allows, you should fly at the highest lift coefficient possible. This means to use all lift enhancing devices at your disposal, which in a climb includes thrust. Flying with full flaps increases drag, which is another reason not to cut thrust.
Now to the formula for the smallest turn radius. This assumes quasi-stationary conditions, so no climb and no inertial effects are included. But even with those restrictions, it shows what the limiting parameters are. We start with the lift equation which now includes the centripetal force to fly a turn with the radius $R$:
$$L = \frac{\rho}{2}\cdot v^2\cdot S_{ref}\cdot c_L = \sqrt{(m\cdot g)^2+\left(m\cdot\frac{v^2}{R}\right)^2}$$
To arrive at a formula for the radius, we need to isolate it on one side:
$$R^2 = \frac{m^2}{\left(\frac{\rho}{2}\cdot S_{ref}\cdot c_L\right)^2-\left(\frac{m\cdot g}{v^2}\right)^2}$$
To simplify this, we can express the speed $v$ by the lift coefficient like this: $$c_L = \frac{m\cdot n_z\cdot g}{\frac{\rho}{2}\cdot v^2\cdot S_{ref}} \implies v^2 = \frac{m\cdot n_z\cdot g}{\frac{\rho}{2}\cdot S_{ref}\cdot c_L}$$
in order to arrive at
$$R^2 = \frac{m^2}{\left(\frac{\rho}{2}\cdot S_{ref}\cdot c_L\right)^2\cdot\left(1 - \frac{1}{n_z^2}\right)}$$
The only variable parameters for a given airplane and height are (besides the radius) the lift coefficient and the load factor. In order to minimize the radius, the parts in the denominator on the right side need to be maximized:
$$R_{min} = \sqrt{\frac{m^2}{\left(\frac{\rho}{2}\cdot S_{ref}\cdot c_{L_{max}}\right)^2\cdot\left(1 - \frac{1}{n_{z_{\:max}}^2}\right)}} = \frac{m\cdot\frac{n_{z_{\: max}}}{\sqrt{n_{z_{\:max}}^2-1}}}{\frac{\rho}{2}\cdot S_{ref}\cdot c_{L_{max}}}$$
So what does this tell us? In order to fly a turn with the smallest possible radius your airplane needs to have a low wing loading $\frac{m}{S_{ref}}$, you need to fly low (high air density $\rho$) and at the highest possible load factor $n_z$ and lift coefficient $c_L$.
Depending on the maximum load factor, speed will still be high compared to that at slow level flight. How high can be seen in a turn rate diagram. It differs from your diagram, but not so much: Use the lines of equal radius (which are radiating away from the origin) as the basis of the Y-axis and distort the result such that those lines become parallel, and you will end up with your diagram.

Turn rate diagram (own work) for a small aircraft. The bold, coloured lines represent the maximum stationary load factor over speed for a given engine thrust at various thrust characteristics over speed. The fine coloured lines are lines of equal load factor and the fine straight, black lines radiating from the origin are those of equal turn radius. It can be seen that the minimum instationary turn radius is achieved at the combination of maximum load factor and minimum speed; however, due to the high load factor the speed is quite a bit higher than the minimum speed in level flight. Also, the available thrust is not sufficient for stationary flight, so this point will incur an energy loss which needs to be made up for by adding sink speed or bleeding off climb speed.

my question is, why, from a physical perspective does the turn radius first decrease as we slow from V$_M$ to V$_S$? Is there a physical explanation for this phenomenon?

As you approach the point of the smallest turn radius from high speed by slowing down, $c_L$ will increase and, being in the denominator, will let the radius shrink. Moving further to lower speeds, away from the point of smallest radius to lower speeds, means that both the lift coefficient $c_L$ and the load factor $n_z$ will decrease, so the term $\frac{n_{z_{\: max}}}{\sqrt{n_{z_{\:max}}^2-1}}$ in the numerator will grow while the denominator decreases. This means that with lower speed R will increase again. The turn radius should be smallest at manoeuvring speed, which you call V$_M$ but should more properly be called $\text{v}_A$. Since your equation expresses the maximum lift coefficient in terms of stall speed at 1g, let me modify my equation accordingly with an expression that is valid for the flight regime at maximum lift coefficient (left of the kink, so to speak):
$$R = \frac{m\cdot\frac{n_z}{\sqrt{n_z^2-1}}\cdot g\cdot v^2}{\frac{\rho}{2}\cdot S_{ref}\cdot c_{L_{max}}\cdot g\cdot v^2} = \frac{v^2}{g\cdot\sqrt{\left(\frac{v^2}{v_{min}^2}\right)^2-1}} = \frac{v^2\cdot v^2_{min}}{g\cdot\sqrt{\left(v^4-v_{min}^4\right)}}$$
where $v_{min}$ is the stall speed in level flight. If you look at the units of the parameters on both sides, you will find that the equation above gives a length unit on both sides whereas that is only true for your corrected equation (which I can confirm as correct). There should be no decrease in turn radius as you move away from the kink.

I Figured out where I messed up in derivation of my initial formula and corrected it (see answer I posted above). I came up with same formula as you did. BTW, in Military, we called this Corner Velocity, ($V_C$), and it was a fundamental concept in Basic Fighter Maneuvering (BFM), and Energy Maneuverability theory.  By the way, I added the derivation of my formula, if you want to check it out.

Chandelle.  If you are not familiar with the maneuver, it is a requirement for the Commercial license and is very useful in such a situation.  It is a climbing 180 degree turn that exchanges airspeed for altitude while decelerating the aircraft down to a slower airspeed, and thus allowing for a tighter turn radius.  Personally I would rather exchange that excess airspeed for altitude rather than pulling the power back, but no advice given here today is necessarily valid for the particular day you are flying, specific aircraft you are flying, weather, temperature, density altitude and severity of the situation you are in.  Yeah, I know, that sounds like a legal disclaimer, but it is really the truth - a chandelle is awesome if you have the room to maneuver in the particular aircraft you are in.  It might be fantastic in a STOL aircraft but perhaps you are in a Baron going 180 KIAS in a narrow canyon and have waited far too long to make your turnaround decision, so in the end, "the PIC is the sole authority for the safe operation of the aircraft".  For what it's worth, practice chandelles to give yourself the skills you need should the need arise.  https://en.wikipedia.org/wiki/Chandelle  NOTE:  What I am discussing is similar to that which is found under "CRUISE FLIGHT – BOX CANYON TURN" located at https://www.mountainflying.com/Pages/mountain-flying/box_canyon_turn.html The general idea is a climbing turn, into the wind, to reduce your progress toward the terrain by slowing your aircraft to increase turn radius while gaining altitude - and I wouldn't recommend slowing below Vx, but again, YMMV.  If you think there is a better method, please respond with that method rather than suggesting "nah, that won't work", and remember, all things are variable - no one can give you a pre-planned suggestion here that will work on the day you are flying, in the aircraft you are flying, with the weather you are in, the density altitude, and the exact distance from the terrain and the airspeed at which you are operating.  If you are already that far behind the aircraft in your planning, best of luck in any escape maneuver you perform.

Oh, and if you have a choice, perform your maneuver into the wind if possible.  This will keep your aircraft the greatest distance from the terrain, as your groundspeed slows as you are turning into the wind.  This speaks to your title topic "turn radius as a function of velocity" in the same manner as the chandelle - not only slowing the IAS but also the GS, which is a factor when staying clear of terra firma.

You can edit your answer rather than adding comments.

If you're at a high enough airspeed to climb out of the canyon, then sure..... Otherwise, this is craziness. the second half of a chandelle involves a continuous gradual reduction in bank angle as airspeed approaches minimum (stall) speed. Turn radius increases dramatically as bank angle decreases.

Fooot - yes, I know.  In some situations, I prefer to comment rather than edit.  Charles - I am not suggesting to perform a chandelle to checkride practical test standards - adapt as necessary as in all maneuvers.  Convert your forward speed to altitude in a climbing turn - position yourself near one edge of the canyon and in a manner that you can turn into the wind.  A climb instantly reduces horizontal progress and is more altitude under your belt, reduces your airspeed and groundspeed and decreases your turning radius, all of which assist in staying away from terrain.  Adapt as necessary.

As I said, Otherwise, this is crazy. Slowing down, by climbing or by reducing power, reduces the lift available. and lift is what you need to turn. Your approach will only make your turn, when you get around to initiating it, bigger and wider and you will hit the wall of the canyon. That is, if you don't stall-spin first.

Also, this answer is testament to the unfortunate prevalence of the misconception that the slower you are, the tighter the turn, from second sentence above: "... aircraft down to a slower airspeed, and thus allowing for a tighter turn radius. ". This is just flat out wrong. Look at my derivation of the true relationship of turn radius vs airspeed above.

Actually, full power, dive, 1/2 loop, roll out (Immelman) might be even better.  One would hope to be clear of terrain at the end of the Chandelle.  A Hammerhead might work too.  Extra power helps here, that's why many "crop dusters" have it.  And yes, slowing down shortens turn radius.

Robert, Then of course, you should just stop, then your turn radius would be zero. The misconception that slowing down shortens your turn radius is based on the mistaken assumption that the "G" in the turn radius formula is independent of the V (velocity) and can be set to whatever you want regardless of the velocity. This is of course wrong. Also, in an aircraft, it is not total G that "turns" you, it is only that component of total G that is aligned in the plane of your turn.

And as to performing an an Immelmann, yes, sure, if you have the airspeed to perform an Immelmann, then sure, go ahead. but how many aircraft have this capability? Even in a high performance military fighter (except for those with thrust to weight ratios in excess of 1:1),  you need a minimum airspeed to perform this maneuver (In the F-4 Phantom it was 450 kts). I didn't think it was necessary to specify that in asking about the "stuck in a box canyon" scenario, I am talking about the the typical GA aircraft, at typical cruise airspeeds, not an F-15/16 or Pitt special at 200kts.

Instead of a chandelle you should use a hammerhead turn. Now the radius is smallest because the turn happens in the vertical at almost no forward speed.

Same considerations apply. If you have the airspeed to do a Hammerhead, you are not in the scenario this question is addressing.  And if you don't, what are you going to do when the wing stalls and you are stalled, 50-60 degrees nose up, still pointed UP the canyon, and the nose drops off?  Also, if you have the airspeed to do a Hammerhead, you can do a Immelmann, or a Cuban Eight.

@Charles Bretana it is good that you pursue this, and distilling out the correct formulas will be a great accomplishment.  From your question you indicate the need to maintain vertical lift as a limiting factor, this may not be the case.  A Cessna 172 can loop if it has extra energy, even if it has insufficient thrust.  Note all the maneuvers:  hammerhead, Immelman, pitch up 90/roll 180/pitch down, Chandelle can be done while decelerating, and completed before too much airspeed bleeds off.

It may be in the end, adding power is actually a good move because 1st, up will get you out of any canyon. If up won't work in time, and aerobatics are out, the Chandelle is best because the prop wash at full power will give at least better rudder control.  A 172 at full power can be flow below 40 indicated (inaccurate at these speeds).  Constant attention to the rudder is required, but it is controllable (done at altitude only).  So for general GA, climbing turn, or hammerhead (after a dive), may be the only choices.  Alot depends on altitude above the canyon floor.

@Robert, as you point out, there are so many variables involved, it is hard (impossible?), to come up with an algorithm that considered every possible option. But up, (climbing), WOULD help, but only to the degree that the turning room available increases, (as the canyon widens as you climb),  faster than your turn radius increases as you slow down. Because your turn radius will definitely INCREASE as you slow down. Which is the point I'm trying to emphasize in this question.

And if you look at the second answer I have posted to this question, I have provided the correct formula, (with it's derivation). I had made an algebraic error in my first derivation of the formula (as posted in the question), which sparked my question in the first place.

@Charles Bretana. I agree your (and Peters) formulas hold for the level turn.  The "canyon" scenario (that we should never get into) does indeed present many solutions.  I will look into trying some lower loading "emergency descent" technique, and also using P factor in the slow turn to get it around.  The formulas (that can be added to programmimg) will be very good for level turns.

@taurus69,  I need to mention that the article you reference is wrong, in that it is completely based on a mistaken interpretation of the turn radius formula R=V2/G. In this formula G is not aircraft G, as the writer assumes. It is radial G, the G in the plane of the turn. And he also ignores the fact that below maneuvering speed G available is proportional to Lift which is a function of V squared. So, therefore, Turn Radius increases as Velocity decreases.

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text:
Which commercial aircraft can perform a parallel engine start?
In various Youtube videos, e.g. this one, one can see that the Boeing 787 is capable of a parallel engine start (both engines are started at the same time). Are there any other commercial aircraft around that allow for a dual (or multiple parallel) engine startup?
What is the reason the Boeing 787 can perform parallel engine starts? What is the difference with other aircraft that cannot perform a parallel startup?

starting an engine require bringing some energy to it (usually through bleed air). if the APU or ground pack cannot provide enough energy to safely start both engine at once, it seems wise to start it one by one. Moreover, one running engine may provide energy to start another one.

The main reason behind this capability is that the Boeing 787 starts its engines entirely on electrical power generated by the APU's VFSG (Variable Frequency Starter Generators) vs relying on APU bleed air to start the engines. Thus there isn't the typical drop in pneumatic pressure caused by the pneumatic starters on regular jetliners.
See 787 No-Bleed Systems for reference.
The Airbus A-340 can start both engines on a wing at the same time (1 & 2 then 3 & 4) as described here.

If I know well also the 747 can start two engines at the same time.

The C-130, or L-382 (commercial variant) can start the 2nd & 3rd engines simultaneously using the combined bleed air of the APU + the first engine.  It was a pretty "gee whiz" trick with limited utility in actual operations, though. Only saw it done a few times in about a decade of flying them. Also don't know if the current J models could do that or not, since their engines are different than what the E and H model aircraft had.

J can start first engine off the APU, then upshift and start subsequent three engines simultaneously.  Requires some extra hands.

I remember being shown round some C-130 simulators back in about 1980 and the instructor said that they could start engines in pairs, I think they demonstrated 2 and 3 then 1 and 4.  They mentioned that this was possible but not normal practice.  I’ve no recollection what the power source was.

Upshift, @Arkhem, you mean you can get a C-130 with a manual transmission? Cool!!! :D

The Embraer 175 can start both engines off APU bleed but I don’t know of any airline that has that in its SOP.

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text:
What will the pilot do if all the airspeed indicators fail?
Are there any alternate instruments to indicate the airspeed in case of pitot tube failures?

Let me guess. They... wing it.

Pitch + Power = Performance. Know two variables and the other can be figured out.

A known pitch trim setting should give you an approximate trimmed off speed though that changes with configuration changes.

"What will the pilot do?"  That depends on the pilot.  In the case of the pilots on AF 447, they, stalled the aircraft and then crashed -- killing everyone on board.  Perhaps you ought to ask "what should the pilot do" as that seems to be what you are curious about.

@wbeard52 What you wrote was the core of an excellent answer.  Why just leave a comment?

Are there any alternate instruments to indicate the airspeed

From SKYbrary:

Reliable Sources of Information
The following information sources, independent of the pitot static
  systems, can provide reliable information for situational awareness:

rpm, and fuel flow, for engine thrust indication (not EPR, which may be unreliable);
Pitch and bank display;
FPV (Flight Path Vector) if available and derived from inertial and not barometric sources;
Radio height when below 2500ft agl;
EGPWS
Stick Shaker - may not always be activated but if it is, it is independently reliable;
Navigation systems can provide ground speed and position information (GPS can also provide altitude information);
Radio navigation aids and RNAV.
ATC, in a radar or ADS-B environment, can provide aircraft ground speed.
If TAS can be determined, a rough approximation of IAS at altitude can be calculated by the fomula: IAS=TAS – (FL÷2) eg 400TAS FL300 =
  250IAS.

Note: Some aircraft systems are configured, as a safety measure, such
  that stick shakers and pushers will not operate if there is
  disagreement between systems. Thus, if the aircraft approaches and/or
  enters a stall, these safety features might not activate. However, if
  the stick shaker does activate, it should, in the absence of clear
  contrary indications, be believed.

I believe this answer can be improved by highlighting or explaining interesting portions, instead of quoting a large paragraph of text.

Some aircraft have angle of attack indicator, but most don't even though they have the sensor and use it for stall warning.
What you normally have is an unreliable airspeed procedure. This generally says that you should, depending on phase of flight, maintain specific pitch, set specified thrust and then adjust for rate of climb/descent while keeping the pitch.
Since aircraft is trimmed for speed, generally the procedure calls for being light on controls and correct mainly with engines. Exception is Airbus which keeps auto-trimming even with speed disagree warning on, so there side-stick controls climb and power controls pitch almost independently.
See also unreliable airspeed procedure for A330.

In the planes I trained in, no.
In my training both for small planes and for gliders I did landings without instruments. You get a feel for the speed and height with experience. Of course a small GA plane or a glider are much simpler than a large airliner.

Depends of the type of aircraft - does it have a secondary pitot, inertial nav etc.  If it does - you're in luck.
Also - "Pitot failure" - that would mostly be clogged total pressure tube (insects . . . .), but could also be static, in which case pressure altitude & rate of climb may be unreliable too.  In other words - have to watch for overall funny behavior.

Second pitot does not help, because you can't tell which one failed. You need third one. Intertial nav is useless, because ground speed and airspeed are related very loosely and you need airspeed. The only other instrument that can help is angle of attack, but while all large aircraft have AoA vanes, few have the actual reading shown in cockpit; usually it is only used for stall warning.

An alternative is getting ATC involved. They could give an estimated speed of the aircraft. Since speed is distance travelled over time, ATC should be able to take the time difference between the two blips on the radar. Knowing the scale of the map in the radar, they can work out your speed.
For example:
If the radar has a scale of 1km represent 1cm on the radar screen, and the blip refreshes every 4 seconds. ATC could measure with a ruler (on the radar screen) the distance between the previous blip to the new blip, say this is 1cm. You know that 1 cm is actually 1 km in real life, and the the time between blips is 4 secs. So the plane has travelled 1km in 4 seconds which is about 250m per second (1000m / 4seconds). 250 m/s equals to about 900 kph. The ATC could give you updates on your speed.


text:
What is communicated between flight crew and cabin crew during flight?
During a typical trans-atlantic flight, can anyone list communication between flight crew and cabin crew?
I am interested in "mandatory", i.e. safety-relevant communication between the flight deck and the cabin, such as the before take-off and before landing notifications from the flight deck to the cabin crew.
What information must be communicated (by law) according to, e.g., ICAO or a major country jurisdiction?

Let's exclude any information regarding passengers, pax health issues, operational delays, meals, anything that is not directly safety relevant.

This question is just too broad.  There are hundreds of different scenarios.  Passengers with connections to make, weather, run out of food/drinks/, problems in the cabin, passenger ill etc etc etc etc.

While I agree that this is a broad question, I do think that a good list of common calls and communications could be compiled, either listing actual call ("doors cross checked") or listing a general category ("passenger meal or drink requests"). I think that such a list could be compiled to reasonable length appropriate for this format.

@JonathanWalters In which case, it would substantially duplicate this question

I'll have the fish please...

The FAA at least would seem not to have any requirements of communication between flight and cabin crews. However, airlines must have guidance or standard procedures for such communication. One thing I have always heard--but cannot find in any regulation--is a PA announcement to the effect of "cabin crew, prepare for takeoff;" I can only assume that all airlines have adopted this.
This is not to say that communication does not occur; quite the opposite in fact. Cabin and flight crews communicate regularly about all sorts of issues, from upcoming weather to passenger issues and operational concerns. It is only that such communication is not required by regulations (at least not in the United States).

Turbulence issues such as severe weather ahead that they need to tell flight attendants to take their seats would be one.


text:
What do the pointers on a Kollsman altimeter indicate?
In this thread: What type of altimeter is this?
there are 3 images of altimeters that have a pointer that moves around the outside diameter of the scale markings and they have another pointer that moves around the inside diameter of the scale markings, e.g 4 and 5 on this picture.

(Source)

What is the purpose of the triangles (4 and 5)?
What are the units or multiples used?

If possible, please give an example of what one of these is indicating. I mean like: so many feet of some kind of altitude.

In each of the examples in that answer, the outer (small) hand indicates thousands of feet and the inner (large) hand indicates hundreds of feet.  Of course, some altimeters indicate metres.  the first example shows 2780 feet, the second 9120 and the third 160.

Sorry for the confusion. I wasn't clear enough about the pointers I was referring to. In the image Federico posted below, I would like to know what the triangles are where the outside one is between the 8 and 9 and the inside one is just to the left of the 0.

I found some sources of definite information, so I will answer this to confirm the suggestion made in a comment that Federico referred to in his answer. Also to fill in the details of what the OP asks.
Triangles 4 and 5 indicate the altitude difference between ISA pressure at sea level and the pressure set in the Kollsman window.
TM 1-205 Air Navigation 1940, Chapter 1, Section VII, Paragraph 52, Altimeter, beginning on page 66, at https://archive.org/details/TM1-205 has an explanation about these triangles, or as they were called "reference markers".
This post at https://www.pilotsofamerica.com/community/threads/blue-and-yellow-does-anyone-remember.79471/ beginning at post #11 discusses altimeters with these markers. Post #25 has a link to an article in the January 1948 issue of Flying Magazine that has a very good explanation of altimeters and these reference markers.
Triangle 5 shows if an altitude difference is higher or lower by which side of the 0 it is on. A higher difference is to the right of the 0 and a lower difference is to the left of the 0. This is in thousands of feet.
Triangle 4 reads in hundreds of feet. Which way you read it depends on which side of the 0 triangle 5 is on.
In the image in this thread, the altimeter setting is 30.1. To see what the difference should be, I used TM 55-6610-247-40 Table 3-1. Altitude Pressure Table, Feet VS Inches of Mercury. I got this TM from http://www.liberatedmanuals.com it says that the altitude difference would be -165 feet.
Triangle 5 is to the left of the 0, between it and -200 feet on the thousands scale. Since triangle 5 is on the left side of the 0, triangle 4 is read from that side and shows -160 feet.
A couple more examples from the thread referenced in the OP What type of altimeter is this?
In the top image the Kollsman window is blanked out. According to TM 1-205 page 68, (3), this is because the limit of the pressure range has been reached. In this case, at the upper limit. The triangles indicate approximately -2380 feet, which would be in excess of 32 inHg.
The bottom altimeter is set to 29.16. According to the table mentioned above, the difference is 711 feet. The triangles are showing this value. Triangle 4 is read from the right side of the zero, because triangle 5 is to the right of the 0.

Taking the image straight from the answer to the question you linked:

(Source)

What do these pointers indicate?
What units do they represent?

It is written in the image: the outer is hundreds of ft, the intermediate is thousands and the inner is tens of thousands.
For example, the altimeter in this image shows an altitude of 9120 ft.
If, on the other hand, you mean the little triangle that is below the "0" in this image, from @mins comment,

it likely indicates the difference between ISA pressure at sea level and pressure set in window, expressed in altitude. It would turn with the setting disc.

What about the little notch on top of the tick mark at 8.4, marked with a "4" in the question?


text:
What is the Correct Procedure to Test Portable Backup Airband Radio (US)?
I recently obtained a portable air band radio for use as a backup radio in flight.  As this is a backup, it isn't much use without being tested.  I'm trying to determine the proper way to test the radio.  The FCC regulations clearly indicate that testing and maintenance is allowed so long as it does not interfere with actual usage of the frequency bands, but here is where it gets tricky, the regulations state that the licensee is allowed to test the radio, but most aircraft in the US no longer require a direct licensee as they don't require an explicit FCC license.
My take on this is that if you had an explicit aircraft station license, you could test the radio equipment even when it is not installed in the aircraft, however it is unclear how this maps to a pilot who has an air band radio they could use in an aircraft, but does not have an explicit aircraft license.
I realize this might be a bit too close to legal advice, but was wondering if anyone had any experience with this and what they found to be the proper procedure for testing the radio.  I would just go and test it on my next flight, but there was also some documentation that requested not doing such tests from the ramp, so it's a bit unclear.

If you're in your plane at a controlled airport, and you call ground or whomever to check comm 1 and 2, why not just call in the same for comm 3?  They're not going to care whether it's a portable or not.

@mins not sure the faa-regulations tag applies, this is more FCC regulations than FAA regulations, though I suppose there is some impact from FAA regulations in that the FCC regulations define "aircraft".

@JohnK - my concern about ramp checking is based from here: https://www.fcc.gov/aircraft-stations.  Under operation it says "Shorten or eliminate test calls on the ramp or in flight."  That might just be trying to encourage it not to be a "check every time" kind of thing, but they don't elaborate.  It's why I'm tempted to lean more towards a radio check on CTAF, though I like the dummy load idea.

"not sure the faa-regulations tag applies": That's not wrong. But there is no FCC tag. Possibly it's best to remove FAA nevertheless (I leave it to you to decide), I apologize for the edit.

@mins no worries, I had debated the same, and unless someone had looked at my network profile, I look like a pretty new user on here. (And I am still pretty new to the aviation stack specifically).  No hard feelings, was just asking for clarification since it wasn't really clear if it was a good fit for that tag or not.

Of course if there is another frequency to use that doesn't involve ATC, like an FBO's unicom, you would use that.

Legally, the FCC’s “license[] by rule” applies to any FAA-licensed pilot in any FAA-registered plane within FAA-managed airspace. There is nothing saying the radio involved must be permanently mounted in the plane.
So, if you wanted to scrupulously comply with the law, just go down to the airport, sit in a plane and do your tests there.
Practically speaking, the FCC doesn’t care what happens on airband unless the FAA complains to them, and the FAA won’t complain unless you’re interfering with the safe, orderly and efficient flow of traffic. For instance, nobody cares if an instructor on the ramp uses a handheld radio to talk to their solo student in the pattern, even though that technically isn’t legal.
That said, if this radio is intended to be an airborne backup, you really do want to test it in an airborne plane. In particular, the metal skin of the plane can act like a Faraday cage, which will drastically reduce your range (unless you have an external antenna connection on the instrument panel), and the range of a handheld typically isn’t great to begin with. You need to see for yourself how bad this effect is in your plane with your radio. Also, learn the proper phraseology for relaying messages to/from ATC via another aircraft.

Good thought about that level of testing.  For the moment I just wanted to make sure the transmitter was working at all, but it's a valid point that range may be impacted by the airplane itself.  That said, even if the range ends up being poor, it should be better than nothing even if it only works in pretty close.  (This particular one is reported to have the best range out of the radios I was able to find reviews for.)

@AJHenderson “Best” in this context means “least horrible”, like claiming the “best” smelling farts. Expect to need an airborne relay.


text:
Where does the final cooling take place in the air cycle machine of the air conditioning system?
On aircraft with an ACM air conditioning, where does the final cooling take place? Is it in the ACM expansion turbine or in the ACM compressor?

Specifically, please elaborate on what kind of air conditioning system you're referring to. Are we talking about airliners here? Single-engine pistons?

Yes,it is about jet engine a/c

Related, the different temperatures in the a/c system. The last cooling occurs at the turbine stage. From this answer.

Related: https://aviation.stackexchange.com/a/13321/4108


text:
Why do the emergency oxygen masks for some 787 premium class seats have an extra 'streamer' to pull them down?
Specifically, on Norwegian 787s, you are briefed:

"In economy seats, reach for your mask. In premium seats, pull the
  streamer for the mask to drop the full distance."

(source)
Now the overheads in premium and economy appear identical:

(Source)
And the premium seats are merely larger versions of economy, they're not beds or fully-reclinable. They're set 2 - 2 - 2 rather than 3 - 3 - 3. I'm guessing there are just two masks in each premium overhead where there would be three in economy, but that doesn't explain the "streamer" (which is just a piece of red cord).
So in premium there's an extra step; of course you end up with exactly the same mask. It puzzles me what the thinking is for the "streamer".

It's interesting that in the safety card the person using the streamer is shown with his seat reclined. And the seats shown in the picture do look like they could recline much further than an economy seat. My guess is that the streamer is there for people who have the seat fully reclined; it's probably too low to reach the mask directly. It would be dangerous to have people panic and try to stand up to reach a mask while there's probably an emergency descent in progress.

Yes good spot. I suspect that's the answer. Some airlines will have more bed-like premium seats than others, so you'll need the streamer if you're nearly horizontal.

So in premium there's an extra step

For what I see in the picture you posted, not really.
There is always the step "grab the mask and pull firmly" to bring it from the compartment to your mouth (and break the supporting cable, that is now unneeded, in the process).
What I understand from the third picture (first row) inside the image you posted is that they made this step "easier" for the premium class, they attached a string/made the usual one longer, so that you don't have to reach up to the mask pulling yourself up, but you can make the mask come down the full distance by pulling the extra string that will come down near to you. Similar to what is shown in this other card found by mins.

I can't see how this is not an extra step though. Economy = grab the mask, Premium = pull the string, grab the mask.

I see where you are coming from, but in the economy version you still have to pull.
I agree that having to grab two objects in a sequence rather than only one seems to be counter-intuitive for an emergency situation, but without seeing the mechanism in action we cannot say much more than this.
The leaflets seem to indicate that the "premium" solution is made to be more comfortable to reach, nothing else.

Thanks for the comments - I can't see how this is not an extra step though. Economy = grab the mask, Premium = pull the string, grab the mask. That's an extra step. In the fear and confusion and panic of a depressurisation event, where people have to react quickly, it's a big call to justify "doing something else".
So, the question is why. The implication is that somehow the overheads work differently. But my observation (as in the question) is that in the 787 they're exactly the same as economy, except where the cabin slightly narrows. But not higher.

Added a pic link as comment to main question to hopefully clarify.


text:
Does the velocity of air in an engine increase in the compressor section or turbine section?
I think it increases in the turbine section because the pressure and temperature decrease so the velocity increases but I'm not sure.

Air velocity increases in the turbine section.
In order to compress air, we need to apply work on it, like running a car engine harder to drive it up the hill. Part of the work required comes from decelerating the air that flows into the engine.
The work done can be released by accelerating the gas and letting it escape, creating thrust. The expanding gas also applies work on the turbine blades, which absorb part of the accumulated energy. Each turbine stage slows down the gas, but from a higher velocity base due to the expansion. In the turbine section as a whole, gas velocity is increased.
The faster the gas flows out of the engine, the higher the thrust that is created.

Actually the velocity of the gas varies throughout the engine.  However the gas expands and actually slows as it passes through the turbine section (it has to as it is converting its internal and kinetic energy into mechanical energy for the turbine.  Gas is accelerated, however, as it passes through the diffuser (nozzle) at the end of the jet pipe.

@Notts90 The incoming air is decelerating now.


text:
A320: Why do SDACs supply all three DMCs?
On the Airbus A320, in the Electronic Instrument System (EIS), there are three Display Management Computers (DMCs):

DMC1 supplies the upper ECAM
DMC2 supplies the lower ECAM
DMC3 is a backup.

There are also two System Data Acquisition Concentrators (SDACs) that acquire system status, process it, and and send system page data to the three DMCs.
Why would it send it to all the DMCs?
The Hydraulic System is displayed only on the lower ECAM which is supplied by the DMC2. Nothing about the hydraulic system is shown on the upper ECAM supplied by the DMC1. So why would SDACs send data to DMC1? I can understand why it would send it to the DMC3 because its a backup but why the DMC1?
Same thing for the Air Conditioning System, Electrical System, etc. None of those systems are displayed on the upper ECAM supplied by DMC1. The only thing displayed on the upper ECAM is engine and MEMO (or emergency stuff in the case of an emergency).
I guess the engine is a system but there is an engine system on the lower ECAM as well. The upper ECAM shows the engine performance and the lower shows the oil, so not the same thing.

I'm guessing here, but it may have the ability to display all the lower ECAM data on the upper ECAM when the other one fails and vice-versa.

So your saying its an extra backup, not such a bad idea.

The Hydraulic System is displayed only on the lower ECAM which is supplied by the DMC2. Nothing about the hydraulic system is shown on the upper ECAM supplied by the DMC1.

That is not true. If DMC1 or DMC2 fails, DMC3 takes over. But if the display fails, the other display takes over. So the hydraulics page can be displayed on the upper display. It can even be displayed instead of the MFD or even PFD if both ECAM displays fail. Similarly for all the other ECAM pages.


text:
What is the purpose of a reverse-flow orientation for turbo-prop aircraft?
If I decide to utilize a PT6 (ignoring for now the cornucopia of variants) as the turbine engine I will be using on my newly designed turbo-prop aircraft, do I then decide how I will orient the engine or is the this something that is engineered into the engine and thus a consideration when deciding which we choose?

Source (adapted)
What compels an aircraft design team to utilize a reverse-flow (turbine ahead) turbine engine orientation? Is the reverse-flow a design of the engine, or maybe an option?
What are the benefits/ drawbacks of reverse-flow designs to the design engineers, mechanics, flight crew, and/or airlines?

So, I knew I should have refined my example, my bad. So the reference to the PT-6 was just a random choice. The question that I should have just asked is "In some turbo-prop aircraft, what is the benefit/ purpose of turning a turbine around so the fresh air intake is facing the tail and the exhaust is facing forward?". I agree with Jonathan, and realize my confusion. While mutually exclusive, are there many with free-wheeling turbo-prop acft with fwd facing turbine engines? Why would they choose that instead of the aft facing engine? Sorry for the poorly stated original question.

Yeah, exactly! Was that you who put that image in? I'm new to this so I don't know how everything works. That was a good image, I liked it, but when I put in the KingAir engine image to show what I was talking about, it  didn't keep the other one for some reason.

Yeah, so what is the purpose of turning it that way? I guess it's facing aft because of the free-wheeling design. With how the exhaust energy is transferred to the propeller, if the engine were oriented intake forward, you'd need to run an additional shaft forward to the propeller gearbox like a turbofan engine. Did I Just answer my own question?

Are you talking about the reverse flow of the combustor or the reverse flow of a PT-6 installed in a tractor configuration (puller)?
The reverse flow design of a PT-6 using a power turbine separate from the gas core offers several advantages:

simplified gas core and power turbine without the additional complexities of nested spools.

modular gas core, power turbine, propeller gearbox and accessory gearbox for ease of maintenance.
design facilitates ease of installation in a pusher configuration e.g. Beech Starship, Piaggio P180 Avanti without complicated and heavy propeller drive trains.
Engine exhaust from a pusher configuration additionally provides propeller deicing without additional electrical or chemical deicing boots.
the reverse flow of the tractor layout allows for additional FOD protection vis a vis an intertial separator prior to air entering the first stage of the compressor.  This makes the engine more adept to operating from unpaved airstrips or ones covered with rocks, ice or other debris.
separate gas core and power sections reduce spool inertia in the gas core, making the core - and the engine - more responsive to throttle inputs from the flight crew or autothrottles.

You are confusing reverse flow with free turbine. They are separate and independent concepts.

@JonathanWalters, they are separate concepts, but definitely not independent—reverse flow only makes sense when free power turbine is used. For single-spool engines, reverse flow does not simplify anything.

@JanHudec The free turbine concept is certainly independent of the reverse flow orientation of certain engine designs, such as the PT6 or Walter. See the PW100 series: direct gas flow with a free turbine. Hence my comment: the free turbine is not a unique advantage of the reverse gas flow orientation.

@JonathanWalters, yes, but that does not make them independent concepts, because reverse flow still exists only to make free power turbine simpler and wouldn't be used without it.


text:
Are the items in newbie "kits" actually used during or after training?
I want to start training for my Private certificate but I do not want to waste my money.  I'm already purchasing an online course for ground school.  Generally speaking, are the following used during or after training?  And, if so, would a $150 US price tag be worth it?

Electronic E6B Flight Computer
FAA Aviation Weather Text Book
FAA Airplane Flying Handbook
Private Pilot Maneuvers Guide
Pilot's Handbook of Aeronautical Knowledge
Pilot's Log Book
FAR/AIM Fuel Tester
Flight Gear HP Crosswind Bag
Sectional Plotter
VFR/IFR Flight Plan Forms

Thanks, in advance.  I'd rather spend my money on flight time than nonsensical items.

All of the FAA handbooks and the AIM are available for free on the FAA website. The FARs are also online. You can download them to your computer or iPad. I like the fact that they are searchable, so you can find things much more quickly than in a book.

Personally, the only things I use these days is the log book, fuel tester, and plotter.  I would probably have used the on-line manuals if there had been an internet when I was a student pilot.  I still have the mechanical E6B I got back then, stuck in a drawer somewhere.  I don't think I've used it since I finished training.

Is it a "FAR/AIM Fuel Tester" or "FAR/AIM" and "Fuel Tester"?

Well going by Sportys own price lists
Flight Gear HP Crosswind bag - 50
PHAK - 16
AFH - 16
AWTB - 16
FAR/AIM - 16
Fuel tester - 11
E6B - 40
Logbook - 16
Plotter - 16
Private Pilot Maneuvers Guide - 16
TOTAL: 213
If the list price is 150, that represents a savings of 63.
Of all that equipment, save for the maneuvers guide, you will need the rest of it to complete your training knowledge test and checkride.  The E6B and the chart plotter are a little anachronistic these days but can still be put to good use for time or fuel sensitive flights.   You will make good use of the flight bag for the rest of your flying career.  All in all it’s a pretty good kit.
One thing they left out and I highly advise you buy is a flashlight and spare batteries.  It’s a $30 item and it could save your life if you ever have an electrical failure at night.  I would recommend the Smith and Wesson Captain’s Flashlight for this.  Avoid tactical flashlights with high output white lights; these are counterproductive when flying and will wreck your night vision.  Another good option are the head mounted lights popular with cavers and climber.  Select one with white/red output and, again, not too bright.
Another useful item is a headset.  High end ANR headsets cost over 1000 USD but ther are economical and good quality one in the 200-300 USD range.  I used the David Clark H10 series headset during my primary flight training and still have the headset handy for passengers.
I would recommend spending the money on the following apps
Either ForeFlight or Garmin Pilot as an EFB.  Best thing since sliced bread for charts, maps, weather products, flight planning, filing, etc.  much lighter and a hell of a lot cheaper than paper products for this.
ASAs FAR/AIM - Heck of a lot lighter than a paper FAR/AIM, can search regulations faster and it automatically updates annually, instead of having to buy a new paper one every year.

To be honest, sliced bread isn't that great for charts.

@Sanchises I'd rather bring sliced bread than a loaf and a sharp knife in a small GA aircraft cockpit...

While the kit may be a bargain, that doesn't really answer the question.  Should I buy all of this?  Also, I plan on buying ForeFlight and an iPad Mini.  So, the question still remains: do I need to buy this stuff?

All and all for a student pilot flight kit, that one is pretty good contains virtually all of your study material and some needed cockpit equipment for less than buying all the parts separately. All in all I’d say it’s a good deal.  But that’s gonna be your call.

@Sanchises It can be useful if you want to make a sandwich while reading the charts, though.

Personally, here are the items I use a lot:

Flight bag
Log Book
Fuel Tester

Items I sometimes use, but you could easily replace

E6B
Sectional Plotter
Flight Plan Forms

Items only used as a student:

FAA Aviation Weather Text Book
FAA Airplane Flying Handbook
Private Pilot Maneuvers Guide
Pilot's Handbook of Aeronautical Knowledge

Assuming I were buying ForeFlight and an iPad Mini, which of the items above should a new student buy?  The entire kit?  None of it?

A good pilot is always a student. I still often read things in the Airplane Flying Handbook, PHAK, AIM and other resources. I got my PPL in 1999, IFR in 2005 and have flown jets and helicopters and there's still so much I don't know.

@dvnrrs: Sure, but I think the question is whether it's worth paying money for hard copy books when you can download an online version for free.


text:
What's the difference between primary, secondary, simplex and duplex nozzles, especially in a PT6?
I'm confused about the difference between the nozzles and the PT6A-114 engine manual is of no help.

Primary vs secondary
When the engine is started:

Fuel is initially distributed to only some nozzles, known as the primary nozzles.

When N1 has reached some threshold, fuel is also distributed to the rest of the nozzles, the secondary nozzles.

In this scheme all nozzles, primary and secondary, have a single orifice (simplex nozzles).

The dual manifold assembly consists of 14 fuel manifold adapters (seven primary, six secondary and one secondary inlet adapter) each of which incorporates a simplex fuel nozzle. (Source: PT6 Training Manual).

The sequence is managed by the Fuel Control Unit and the Starting Flow Control.
Simplex vs duplex
A simplex fuel nozzle has one orifice, a duplex nozzle has two orifices:

Duplex nozzle on the left, simplex on the right (source). Duplex nozzle from the PT6 Training Manual:

Using duplex nozzles, the starting sequence is different, all nozzles are fed, but:

Only one orifice is used before N1 threshold (primary flow)
The second orifice is used after N1 threshold (secondary flow).

This way all nozzles are always used, the fuel spray pattern and the flame are more homogeneous in the annular combustor.


text:
What are the adverse effects of keeping the PACKs on the A320 running with the LP air conditioning unit connected?
I had come across this statement will reading the limitations chapter in the FCOM. But once again there is no reasoning given for it I have also read the air conditioning systems chapter but can't seem to find an explanation.
Can someone explain the reasons?

Latest FCOM, after revision 24 of section 3.01.21, contain the explanation:

AIR CONDITIONING WITH LP GROUND UNIT

Do not use conditioned air simultaneously from packs and LP ground unit (to avoid chattering of the non return valves).

The non-return valves prevent air in the mixer unit to return into any source in the case this source delivers air at a lower pressure, or doesn't deliver air. Valves are the one-way separation between the air generation section and the air distribution section:

 Source

If the LP ground connection is used while the packs are ON, pressure will be comparable on both sides of the valves. Valves can start chattering instead of being stable in the open position. While pressures compete, air from the mixer unit can return to the packs.
Damage can occur, as described in this 1991 incident (air conditioning duct and non-return valve damaged).

"In some FCOM" or "In the same FCOM"? I'm not sure if your quote & picture are specific to the A320 or are generic. (Also, please link sources! :)

More than good enough for me, sir! You might want to note that in the answer, just in case someone gets carried away with comment clean up.


text:
What does "Mods" and then the check marked list of numbers mean on avionics?
Most of the avionics I see have a label that says "Mods" and then various numbers checked 1-15.

Image source.
What does this information relate to?

This indicates the "mod level" of the hardware in the unit. When a piece of equipment is designed and then certified, it is likely that none of the boxes will be marked.
Then say the product design has a hardware change made to it. Whatever that change consists of is then incorporated into new production units at "mod level" 1. Any units still out in the field won't have this change unless they are reworked to include the modifications. Once the rework is done, the mod plate is then updated to mark the "1" box.
Some installations may require all versions of that equipment to be at a certain mod level in order to meet the certification requirements of that installation.

Pretty much. I think some equipment may not have all of the mods. Like having mods 1, 2, and 4 if mod 3 wasn't required for whatever reason, but that can be a big headache to manage from a unit configuration standpoint

Each manufacturer will do it differently as well.  Often the mod state is a letter suffix after the Part No or Serial No.

"Mod" comes from modification. it is also common to refer to different mod levels as revs, short for  "revision level" as in rev 1 (as issued), rev 2, rev 3, rev 4 as more revisions are incorporated into the original design.


text:
Why does a flight booking mention that the plane has wingtips?
Often when booking flights the plane is listed as having wingtips.  For example, it would say 737 (wingtips).  I get what the wingtips are for, but what is the purpose of telling the passengers that the plane has them at time of booking?

What airline are you seeing this for?

@RonBeyer KLM's (for example) fleet list used to list those 738s that had been retrofitted with winglets as such when this retrofitting program was underway. When the program was complete, the information was dropped. This would show up on flight schedules as something like "Boeing 737-800 (winglets)".

This is nothing but a silly marketing exercise. Wingtips (or rather winglets: all airplanes have wingtips:) are seen by the public as a cool and very visible feature, characteristic of newer models. So perhaps they want to say that our airplane is new, and not the 1968 model with the same designation.

I doubt it's even really meant as a marketing feature so much as the database of aircraft types happens to identify the aircraft that way, and that's what ends up getting displayed to the passenger unintentionally.


text:
What are the key differentiating factors between the flight control systems employed in unmanned drones and those utilized in piloted aerial vehicles?
"I've come across open-source projects like ArduPilot that are commonly used with microcontrollers like Pixhawk for unmanned aircraft. However, I'm unsure whether these systems can be adapted for use in manned aircraft. My knowledge of control algorithms in manned aviation is limited to basic concepts like PID controllers. I would greatly appreciate it if someone could kindly provide insights into the control algorithms employed in manned aircraft and share guidance on developing communication protocols for such systems. Your expertise in this area would be immensely helpful. Thank you.

Simple aircraft use pushrods and cables for their flight controls.
Manned aircraft using fly-by-wire controls do so with extremely sophisticated software that include carefully verified algorithms and extensive backups -- not at all the stuff of "limited knowledge of basic concepts."
You're going to need a lot more knowledge than can be gained on an internet forum before you're ready to "develop communications protocols for such {manned} systems." There are discussions on Av.SE of fly-by-wire designs and certification requirements, but this question as it stands is far too broad.

There's not a single computer on a transport aircraft. Navigation is not the same than autothrotle or engine control (FADEC), and control surfaces have their own controllers which use different "laws" in case of failure. Some of these computers run on real-time OSes. So which function do you want to compare to Arduino application? Related: Airbus control architecture - where does the actual autopilot live?

The design configuration of an octocopter is supported by Ardupilot. There's also a single-seat manned aircraft called the Jetson One, which has a design that resembles an octocopter. As a result, the control surfaces on both of these aircraft may appear similar. Then, why not use open-source software, given that the configuration is supported by these software packages?

The primary difference is liability, which drives a chain of other items.
There is an expectation of the various parties involved in approving, licensing and insuring aircraft that there is a clearly designated chain of responsibility to the various decisions and actions should something turn out to be wrong.
So it would be perfectly feasible to take an open source autopilot and slap it into a full sized aircraft registered for experimental use, and quite possibly it would not immediately crash*. However if you want to operate outside the experimental limitations (actually have a useful aircraft) you will need to convince various parties that your autopilot is safe. And saying 'I downloaded software built by strangers from the internet' will not go very far.
The process from here would be basically a line by line effort documenting and explaining how the software worked and makes decisions, and how those decisions maximize safety. This is probably easier than building an autopilot from scratch but still a multi man year project. At the end of it you would be taking liability for any unfortunate outcomes of the system working as designed, which possibly should give you pause if you have had much involvement in crowd sourced software.

Some reasons why a model aircraft autopilot might sorta work but run into trouble entrusted with a full sized aircraft without re-design include:

Different aerodynamic behavior due to scale so various
constants will be wrong.

Square cube law will mean that controls will respond slower/have more
inertia than at model scale leading to induced oscillation.

Square cube law also means structure safety and power margins are
lower, so less aggressive control actions required to avoid mid air
destruction.

Unmanned vehicles detecting failure should find a safe place to crash
ASAP to protect those on the ground. A manned aircraft probably
should not still have 'crash now' code in it or at least very
carefully tuned.

Handling disputes between humans and autopilot for control.

To achieve higher reliability redundancy is involved which means programming systems to read from duplicate sensors, and have multiple controllers with some sort of arbitration managing which actually pilots the vehicle. Dealing with noisy/stale/lying data in aircraft is a still a hard problem.

And saying 'I downloaded software built by strangers from the internet' will not go very far